The setup consists of a 20 m optical cavity with the mirrors on two separate optical tables as shown in Figure REF . The end mirrors are two inches in diameter with a radius of curvature of \(23.2\pm 0.5\)  m giving the cavity a nearly confocal configuration. The transmissivity of the mirrors was measured to be \(878\pm 2\)  ppm. The resonant Eigenmode of the cavity that is used for stabilizing the laser frequency is the Hermite-Gauss \(U_{00}\)  mode defined in [1]}. It has a beam waist of \(1.97\pm 0.02\)  mm and a mode radius of \(2.62\pm 0.01\)  mm at the end mirrors. For a loss free cavity the calculated Finesse is \(3577\pm 8\)  and the linewidth is \(2097\pm 5\)  Hz. This corresponds to a storage time of \(151.8\pm 0.3\)  \(\mu \) s.
<FIGURE>